1. Field of the Invention
The present invention relates to a soft template with an alignment mark and a manufacturing method thereof, and more particularly, to a soft template having alignment marks formed at both sides of a pattern to facilitate forming a fine pattern in a large area, and a manufacturing method thereof.
2. Description of the Related Art
Photolithography is one of the core parts of semiconductor pattern transfer technology. Electron-beam lithography and X-ray exposure are used in manufacturing a photolithography mask and forming a fine pattern. As the semiconductor patterns become finer, the costs associated with the fabricating semiconductor patterns increase.
Nano-imprint lithography (NIL) has been highlighted as an efficient and economic process for forming patterns. NIL has been suggested to embody a nano process (1-100 nm) by transferring a pattern directly from a mold (template pattern) to a substrate by pressing the mold. Thermoplastic resin or photosensitive resin is coated on a substrate, and a nano-sized mold is pressed onto the substrate to transfer a pattern from the mold to the substrate, following by curing using an E-beam. NIL makes it possible to form a complicated step pattern on a substrate in a simpler way than the conventional photolithography. Conventional photolithography requires several processes to form a multi-step pattern, but NIL allows the formation of such multi-step pattern via an one-time press transfer. Thus, NIL is effective, especially for the formation of a multi-step shape pattern. It was reported that NIL could be used to manufacture electronic and optical devices such as MOS-FETs, in replacement of the conventional lithography. However, although NIL may produce a fine structure of a nanometer-scale, it cannot attain the positional accuracy of the alignment, which is provided by photolithography.
The NIL may be divided into two types: hot embossing or thermal imprint lithography, and UV-assisted imprint lithography. In the hot embossing or thermal imprint lithography, heat is applied to a polymer layer to make it flexible, followed by pressing the polymer layer against a mold to form a desired pattern on the polymer layer. However, the application of high heat and pressure requires an expensive, complicated apparatus. Also, there are disadvantages, for example, it is difficult to apply the hot embossing or thermal imprint lithography to form large scale patterns, as it requires a high pressure, and cracks can be formed in the substrate.
To solve these problems, UV-assisted imprint lithography, which uses a polymer cured by UV rays, has been introduced. This method is similar to the conventional hot embossing method, except the former uses a liquid having a viscosity similar to that of water, which is cured by UV rays, and thus pressing may be performed at much lower pressure than the hot embossing. Since UV rays are used to cure a polymer layer, a quartz based mold is used. This method is advantageous in comparison with the conventional imprint method in terms of the process conditions such as lower pressure. However, it is very difficult and expensive to form a nano-pattern on a quartz mold. Also, it is difficult to prevent bubbles, which cause defects, from being trapped in the mold. Presently, these two imprint methods are used in various fields.
Various factors attribute to the advantages or disadvantages of the NIL process. They include, for example 1) enabling a large area process, 2) minimizing a polymer residual layer, and 3) securing a precise alignment. Also, the separation of the mold from the polymer layer and lowering the pressure and temperature are important factors.
As to the factor of the large area process, it has been reported that about a 4″ wafer process could be successfully performed, almost without defects. For a larger size substrate, there is currently no report of successful overall pattern transfer of a wafer by one-time process, probably because applying a uniform pressure over a large area is difficult, and the price of the mold itself is high. Thus, a “step and repeat method,” in which a small mold is repeatedly printed, has recently been widely used. In theory, the step and repeat process can be applied to an 8″ or larger wafer.
The residual layer-related concerns are unequivocally applicable to molding processes, including imprint processes. When the polymer is pressed by a mold, a polymer residual layer having a certain thickness always remains on the substrate. Thus, two-step dry etching including a polymer etching process is needed to remove the polymer residual layer. Much research is directed towards reducing the polymer residual layer.
A method using a soft template instead of a rigid template has been suggested to enable a large area process and minimize the polymer residual layer. FIG. 1A illustrates a conventional nano-imprint process using a rigid template, and FIG. 1B illustrates a conventional nano-imprint process using a soft template.
Referring to FIG. 1A, a resin layer 12 is formed on a substrate 11 having an undulated or wavy surface. A pattern 12a is transferred to the resin layer 12 by applying pressure using a rigid template 103. When the rigid template 103 such as quartz is used, an irregular pattern may be obtained and a polymer residual layer may be left, limiting the use of the process for the formation of a large area pattern.
Referring to FIG. 1B, a soft template 13 can transfer a pattern for the resin layer 12 onto the undulated substrate 11, so that a uniform pattern 12b can be obtained and the polymer residual layer can be minimized. The soft template 13 can be manufactured through a UV nano-imprint process, at a low cost, from a master template formed of quartz or Si.
The alignment-related concerns are relatively less frequently faced in an exposure process where a master pattern and a substrate are not in contact to each other. The exposure process can be easily applied to a curved or multilayered structure. However, when the nano-imprint process is applied to a curved or multilayer structure, the chances of failing to transfer a pattern increases. Many studies to solve this problem using an alignment mark are underway. For the soft template, no solutions applicable to the nano-imprint process have been suggested.
The conventional nano-imprint process uses a method of forming an alignment mark on the surface of a template by etching. However, when the template is used, it is difficult to identify the alignment mark when the patterning resin is bring into contact with the template during the imprint process, because the template and patterning resin usually have similar refractive indexes. To solve this problem, as shown in FIGS. 2A through 2E, a method of forming an alignment mark in a lower portion of a pattern of a template has been suggested.
Referring to FIG. 2A, resist patterns 22 are formed on a glass or SiO2 substrate 21. As shown in FIG. 2B, reactive ion etching (RIE) is performed to the substrate 21 in a pattern area 23 between the resists patterns 22. Referring to FIGS. 2C and 2D, an alignment mark 24 and SiO2 are deposited, and then RIE is performed again. Referring to FIG. 2E, the resist 22 is removed in a lift-off process to obtain an alignment mark embedded template. It can be seen that the alignment mark 24 is formed in the lower portion of each pattern area 23. However, for the template shown in FIG. 2E, it is almost impossible to identify the nano-sized alignment mark 24 that is formed in the lower portion of the pattern 23. To identify alignment mark, the alignment mark needs to have a size in micrometers. Thus, it is difficult to be used for a nano-sized imprint process.